In medium- or high-voltage electrical appliances, the electrical insulation and, if appropriate, the extinguishing of the electric arcs are typically provided by a gas which is confined inside a chamber of these appliances. Currently, the gas which is most often used is sulfur hexafluoride (SF6): this gas exhibits a relatively high dielectric strength, a good thermal conductivity and relatively low dielectric losses. It is chemically inert and nontoxic to man and animals, and, after having been dissociated by an electric arc, it rapidly and virtually completely recombines. Furthermore, it is nonflammable and its cost is, even today, moderate.
However, SF6 has the major disadvantage of exhibiting a global warming potential (GWP) of 22 800 (relative to CO2 over 100 years) and a residence time in the atmosphere of 3 200 years, which places it among the gases having a high greenhouse effect power.
Manufacturers are thus looking for alternatives to SF6. Hybrid systems have been provided, which systems combine gas insulation with solid insulation (document EP 1 724 802). However, this increases the size of the electric appliances, in comparison with that allowed by insulation with SF6, and the cutting in oil or vacuum requires a recasting of the items of equipment.
The use is known, alternatively to SF6, of “simple” gases, such as air or nitrogen, which do not have a negative impact on the environment. However, these exhibit a much lower dielectric strength than that of SF6; their use for the electrical insulation and/or the extinguishing of electric arcs in high-voltage/medium-voltage appliances involves drastically increasing the volume and/or the filling pressure of these appliances, which runs counter to efforts which have been made in recent decades to develop compact electrical appliances which are increasingly less bulky.
Perfluorocarbons exhibit, generally, advantageous dielectric strength properties but their GWP typically comes within a range extending from 5000 to 10 000.
Other promising alternatives from an electrical and GWP characteristics viewpoint, such as trifluoroiodomethane, are categorized among category-3 carcinogenic, mutagenic and reprotoxic substances, which is totally unacceptable for use on an industrial scale.
Mixtures of SF6 and of other gases, such as nitrogen and nitrogen dioxide, are used to limit the impact of SF6 on the environment: see, for example, the document WO 2009/049144. Nevertheless, as a result of the high GWP of SF6, the GWP of these mixtures remains very high. Thus, for example, a mixture of SF6 and nitrogen in a ratio by volume of 10/90 exhibits a dielectric strength in alternating voltage (50 Hz) equal to 59% of that of SF6 but its GWP is of the order of 8000 to 8650. Such mixtures thus cannot be used as gas having a low environmental impact.
The document FR 2 955 970 provides for the use of fluoroketones in the gas state for electrical insulation. Fluoroketones can be combined with a carrier gas or dilution gas (for example nitrogen, air, nitrous oxide, carbon dioxide, oxygen, helium and the like).
The document FR 2 975 818 provides a mixture of octofluorobutan-2-one and carrier gas as insulation medium.
The document FR 2 983 341 provides for the use of polyfluorinated oxiranes as electrical insulation and/or extinguishing gas for electric arcs.
The document FR 2 986 192 provides for the use of a combination of poly fluorinated oxirane and hydrofluoroolefin as electrical insulation gas. The hydrofluoroolefins mentioned are 1,3,3,3-tetrafluoropropene (HFO-1234ze), 2,3,3,3-tetrafluoropropene (HFO-1234yf) and 1,2,3,3,3-pentafluoropropene (HFO-1225ye).
The document WO 2012/038443 provides for the use of a mixture of SF6 and fluoroketone as electrical insulation gas.
The document WO 2012/160158 provides for the use of a mixture of decafluoro-2-methylbutan-3-one and a carrier gas as electrical insulation gas.
The document WO 2013/004796 provides for the use of a gas based on hydrofluoroolefin as electrical insulation gas. The hydrofluoroolefins more particularly provided are 1,3,3,3-tetrafluoropropene (HFO-1234ze) and 2,3,3,3-tetrafluoropropene (HFO-1234yf).
The document WO 2013/041695 provides for the use of a mixture of hydrofluoroolefin and fluoroketone as electrical insulation gas. The hydrofluoroolefins more particularly provided are 1,3,3,3-tetrafluoropropene (HFO-1234ze), 2,3,3,3-tetrafluoropropene (HFO-1234yf) and 1,2,3,3,3-pentafluoropropene (HFO-1225ye).
The document WO 2013/136015 provides for the use of a mixture of hydrofluoroolefin and hydrofluorocarbon as electrical insulation gas. The hydrofluoroolefins more particularly provided are 1,3,3,3-tetrafluoropropene (HFO-1234ze), 2,3,3,3-tetrafluoropropene (HFO-1234yf) and 1,2,3,3,3-pentafluoropropene (HFO-1225ye). The hydrofluorocarbons more particularly provided are 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea), pentafluoroethane (HFC-125) and 1,1,1,2-tetrafluoroethane (HFC-134a).
There still exists a need to develop electrical insulation and/or extinguishing media for electric arcs exhibiting both a low GWP and exhibiting a high dielectric strength.